Magnesium is the eighth most abundant element in the earth's crust but does not occur uncombined in nature. It is found in abundance in the minerals brucite, magnesite, dolomite, and carnalite. It is also found (as the silicate) in asbestos, meerschaum, serpentine, and talc. Magnesium chloride is found in seawater, brines, and salt wells. Mineral waters often contain salts of magnesium; the magnesium ion imparts a bitter flavor. Magnesium is a constituent of the chlorophyll in green plants and is necessary in the diet of animals and humans.